Field of the Invention
The present invention concerns a method for determining a physiological activity signal in a subject using a system having several movement sensors. The invention further relates to the associated system for determining the physiological activity signal and a storage medium encoded with programming instructions for implementing such a method.
Description of the Prior Art
Knowledge of respiratory activity and heartbeat activity are of fundamental importance for imaging such as for an MR system or CT system, in order to be able to prevent motion artifacts because then recording of the measurement data in the MR or CT system can be synchronized with the heartbeat activity or respiratory movement by triggering. Heartbeat activity is usually measured by means of EKG detection, but this is time-consuming, because each EKG electrode has to be attached to the ribcage. Furthermore, methods for the detection of heartbeat and respiration with the use of radar sensors are known, as described in exemplary fashion in the following publications:    1. “Monitoring respiratory and cardiac motion in CT using a continuous wave Doppler radar” (F. Pfanner, T. Allmendinger, T. Flohr, M. Kachelrieβ)    2. “Cardiac and Respiratory Monitoring through Non-Invasive and Contactless Radar Technique” (M Varanini, P C Berardi, F Conforti, M Micalizzi, D Neglia, A Macerata)    3. “Radar monitoring of heartbeats and respiration” (Øyvind Aardal)    4. WIRELESS BIO-RADAR SENSOR FOR HEARTBEAT AND RESPIRATION DETECTION” (B.-J. Jang, S.-H. Wi and J.-G. Yook, M.-Q. Lee, K.-J. Lee)
Furthermore, methods are known which detect the heartbeat (or EKG) with the aid of radar sensors:    5. Microwave Doppler Radar for Heart Beat Detection Versus Electrocardiogram: A Validation Approach (Dany Obeid, Sawsan Sadek, Gheorghe Zaharia, Ghais El Zein)    6. Non Contact Heart Monitoring (Lorenzo Scalise)    7. Wearable Doppler Radar with Integrated Antenna for Patient Vital Sign Monitoring (Richard Ribón Fletcher, Sarang Kulkarni)    8. A Comparison of Continuous Wave Doppler Radar to Impedance Cardiography for Analysis of Mechanical Cardiac activity (J. A J. Thijs, J. Muehlsteff, O. Such, R. Pinter, R. Elfring, C. H. Igney)
FIG. 1 shows a diagrammatic view of how several transmitting and receiving antennas can be used, which have a fixed spatial arrangement. This system of the prior art has a radar unit 10 with several transmitting antennas 11-14 and a receiving antenna 15. Likewise it is conceivable that the antennas are alternately transmitting and receiving antennas. Problematic with all these devices and methods is that respiratory signals and signals from heartbeat activity overlap and can only be separated with difficulty. This is because radar sensors detect movements and represent these movements as signals. As heartbeat and respiratory movement occur simultaneously, both activities also overlap in the resulting radar signal. When it is an uninterrupted respiratory signal that is of interest, the heartbeat activity that is additionally visible in the signal may be disturbing. Likewise, when it is the heartbeat signal that is of interest, the respiratory movement additionally visible in the signal can be disturbing. Both signals, heartbeat and respiratory movement, are therefore each available only in a disturbed fashion as a result of known radar arrangements. However, this means that use as a trigger signal for heartbeat activity or respiratory activity is barely or only imperfectly possible, leading to a deterioration in image quality. Furthermore, a disadvantage of radar arrangements according to the prior art is that due to their fixed position, they are not optimally positioned for all subjects, from large adults to children, from slim to fat patients, with the consequence that the quality of the signals generated fluctuates greatly depending on the patient being examined.